The GT series are sim racers very technical, their not arcade racers like Burnout 3:Takedown. This games takes a lot of concentration, technique and skill to master. Gran Turismo 4 is the latest one in the series as the best one, and hardest one. Too @#$@!#$ bad it did not come with online support (like it matters I don't have broadband, but my buddy does). Over all I love the game but don't let the number of cars foul you their probably like 50 different types of skylines, Subarus etc. But so far my favorite sim racer love goes out to Forza Motersport because not only does it give a solid sim feel but it has more options and more physics (like the cars take damage and even effects the car during the race).
Gran Turismo 4
Gran Turismo 4 (GT4) is the most realistic and comprehensive game of the series, with the greatest number of tracks and cars available. There are at least 700 different cars in the game (although it should be pointed out that this list includes cars with multiple small variations, such as 20 Subaru Imprezas, 25 Mitsubishi Lancers, and 48 Nissan Skylines) and over 50 racetracks. Features new to the series include the ability to install aerodynamic spoilers to increase downforce, the ability for tuned or racing cars to spit flames from unburnt fuel through their exhausts, and the controversial inclusion of nitrous oxide. There are also new Photo and B-spec Modes to cater for a wider variety of car enthusiasts.
The game was released for PlayStation 2 on December 28, 2004 in Asia, February 22, 2005 in the United States and March 9, 2005 in Europe. A version for PlayStation Portable, titled Gran Turismo 4 Mobile (or GT4 Mobile), is currently in development. An online component, originally intended, was removed. However, while there is a lack of online features, the NTSC version of the game has the option of being displayed in 1080i HDTV resolution. It is the first game for the PlayStation 2 that can be displayed in this resolution—and one of only a handful of games available for any current console to do so. The final version was released in DVD-9 format.
In addition, the physics and AI have been completely reworked. The AI for the opposing cars can now be adjusted by the user with up to 10 settings in B-spec mode. GT4 is also the first game in the series to model the drivers of the vehicles, which allows for convertibles to be driven with the top down, and the inclusion of antique automobiles such as open-top early Daimler-Benz Motor Wagen and Ford Model T.
Also, pickup trucks (like the Ford F150 SVT Lightning and Dodge Ram) were featured for the first time on GT4, and new models were added, like the 1987 Buick GNX and both the 1964 and 2004 versions of the Pontiac GTO.
The most significant improvement of GT4 over previous offerings, for most, has likely been the evolution in the physics. Players must learn even more about the subtleties of real life racing in order to be able to drive effectively in the game compared to previous racing games. However, traction and stability controls remain available to provide realistic driving aids for relative beginners while they learn by completing various licence tests.
Licence tests have been an integral part of GT games. Completing higher levels of tests will let the player gain entry to harder races, where victory earns more prize credits and faster, rarer prize cars. This in turn gives the player a better chance of winning the numerous other races as, the money earned will allow for upgrading existing cars in the garage, or buying faster and more expensive cars.
An advanced player will be able to judge his performance by the realistic lap times. According to the developers, a professional driver was invited to set times using the same car on the Nurburgring Nordschleife circuit, and the GT4 lap times were within 2% of the real life equivalent
source this is a wiki page it will talk more about the whole series and at the buttom it even has something of part 5 for the ps3